The emergence of extreme ultraviolet (EUV) projection lithography has placed stringent demands on interferometric metrology systems. In order to achieve diffraction-limited performance, EUV lithographic systems require wavefront tolerances on the order of 0.02 waves rms (0.3 nm rms at a wavelength of 13.4 nm)..sup.1 While the accuracy of interferometry is typically limited by the quality of the reference surface or wave, a class of interferometers has been developed in which extremely high quality reference waves are created by diffraction from small apertures..sup.2-5
EUV lithographic systems rely on wavelength-specific reflective multilayer coatings. To accurately probe phase effects in these resonant reflective structures, at-wavelength metrology is required. Various at-wavelength interferometric measurement techniques including lateral-shearing interferometry,.sup.6 Foucault and Ronchi testing.sup.7 have been reported. These methods, however, have yet to demonstrate the accuracy required for the development of EUV lithographic imaging systems. In order to meet the accuracy challenge, an EUV-compatible diffraction-class interferometer, the phase-shifting point diffraction interferometer (PS/PDI), was developed by Medecki et al..sup.8, 21 The reference wavefront accuracy of the PS/PDI has been demonstrated to be better than .lambda..sub.EUV /300 (0.045 nm) within a numerical aperture of 0.082..sup.9
The PS/PDI is a variation of the conventional point diffraction interferometer in which a transmission grating has been added to greatly improve the optical throughput of the system and add phase-shifting capability. In the PS/PDI, as illustrated in FIG. 1A, the optical system 2 under test is illuminated by a spherical wave 5 that is generated by an entrance pinhole 6 in a mask 4 that is placed in the object plane of the optical system 2. To assure the quality of the spherical-wave illumination, pinhole 6 is chosen to be smaller than the resolution limit of the optical system. Grating 8 splits the illuminating beam 5 to create the required test and reference beams 10 and 12, respectively. A PS/PDI mask 20 is placed in the image plane of the optical system 2 to block the unwanted diffracted orders generated by the grating 8 and to spatially filter the reference beam 12 using a reference pinhole 16. The test beam 10, which contains the aberrations imparted by the optical system, is largely undisturbed by the image-plane mask by virtue of it passing through window 14 in the PS/PDI mask 20 that is large relative to the point-spread function (PSF) of the optical system under test. The test and reference beams propagate to the mixing plane where they overlap to create an interference pattern recorded on a CCD detector 18. The recorded interferogram yields information on the deviation of the test beam from the nominally spherical reference beam. FIG. 1B depicts a PS/PDI mask 21 comprising a square shaped window 22 and a reference pinhole 24 to the right which has a diameter of less than 100 nm.sup.8, 10, 11, 21.
The original PS/PDI.sup.8 requires the image-plane beam separation to be sufficient to prevent the reference beam from passing through the test-beam window. For a given separation, this requirement places limits on the scattering and aberrations that can be present in the optic under test. If these limits are not met, the accuracy of the PS/PDI is compromised. It is not feasible to simply increase the image-point separation to strictly meet the above separation requirements due to the unreasonable fringe density this would produce.
Failure to meet this criterion leads to one of the major drawbacks of the PS/PDI: susceptibility to scatter or high-frequency features that can cause confusion of the test and the scattered-reference beams. In the presence of this scattered light, the reference beam is no longer a clean spherical wave, but includes high-frequency features that make the interferometry more difficult. Mid-spatial-frequency features in the wavefront of interest are especially susceptible to this problem.
For lithographic printing using next-generation projection lithography, it is important to consider flare in addition to wavefront error. Flare is the halo of light surrounding the optical system point-spread function (PSF) that is caused by scatter from within the optical system. The grating beamsplitter implementation of the EUV PS/PDI creates multiple image points in the image plane, and can significantly complicate flare measurements attempted using conventional interferometric analysis techniques. For this reason and the drawback described above, the PS/PDI as originally implemented was incapable of accurately measuring the extended spatial-frequency band required to characterize flare.
Prior art system-level at-wavelength flare testing for EUV optics involved printing. Recently, an EUV scatterometry-based method was described in E. Gullikson, et al. "EUV scattering and flare from 10.times. projection cameras", in Emerging Lithographic Technologies III, Y. Vladimirski, ed., Proc. SPIE, 3676, 717-723 (1999) and E. M. Gullikson, "Scattering from normal incidence EUV optics", in Emerging Lithographic Technologies II, Y. Vladimirski, ed., Proc. SPIE, 3331, 72-80 (1998).
At-wavelength flare predictions for EUV imaging systems were based on scatterometry measurements of the individual mirror elements prior to assembly. This method has the disadvantage of not being a system-level test. Furthermore, the test is not complete because it cannot be performed over the entire spatial-frequency band of interest. Scatterometry requires the use of white-light interferometry and/or other methods to supplement the data. The scatterometry method is not well suited to the measurement of low-spatial-frequency scattering. The art is in need of a system-level at-wavelength technique that can be performed in parallel with wavefront characterization.